Glare and transparency
To see the glare in action, I would do this:



So the glare is working, but it doesn't affect the alpha channel. To make it do so, I'm setting the mix value to 1 to make the node output only the glare. I'm adding the glow to the image and the alpha, so it will be part of the alpha channel too. I'm then recombining them with a set alpha node.

Remember to clamp the add on the alpha, as it should never be greater than 1 (or smaller than 0)

I hope this helps you :D